CN1744326A - Epitaxial substrates and semiconductor elements - Google Patents

Abstract

In a Schottky diode 11, a gallium nitride support base 13 includes a first surface 13a and a second surface 13b opposite from the first surface and has a carrier concentration exceeding 1 x 10 18 cm -3 . A gallium nitride epitaxial layer 15 is disposed on the first surface 13a. An Ohmic electrode 17 is disposed on the second surface 13b. The Schottky electrode 19 is disposed on the gallium nitride epitaxial layer 15. A thickness D1 of the gallium nitride epitaxial layer 15 is at least 5 microns and no more than 1000 microns. Also, the carrier density of the gallium nitride epitaxial layer 15 is at least 1 x 10 14 cm -3 and no more than 1 x 10 17 cm -3 .

Description

Epitaxial substrates and semiconductor elements

technical field

The present invention relates to epitaxial substrates and semiconductor elements.

Background technique

A PIN diode is described in Appl. Phys. Lett., Vol. 83, No. 11, Sept. 15, 2003 by Y. Irokawa et al. PIN diodes are equipped with epitaxial layers grown on a GaN free-standing substrate. Thick films used as GaN free-standing substrates were grown on Al ₂ O ₃ substrates by hydride vapor phase epitaxy (HVPE). A laser is applied to this thick film, which separates it from _{the Al2O3} substrate, forming a GaN free-standing substrate. On the GaN free-standing substrate, a non-doped gallium nitride film with a thickness of 3 microns is grown by metal-organic vapor phase epitaxy. A Mg-doped gallium nitride film was then grown to a thickness of 0.3 microns on the non-doped gallium nitride film. A GaN independent substrate, an undoped gallium nitride film and a Mg-doped gallium nitride film form a PIN structure.

Gallium Nitride pn junctions are characterized in Appl. Phys. Lett., Vol. 73, No. 7, Aug. 17, 1998 by P. Kozodoy et al. First, a GaN film with a thickness of ₂ μm was grown on a c-plane sapphire substrate by metal-organic vapor phase epitaxy using a SiO2 mask for LEO recombination. The mask was formed as stripes with 5 micron openings at intervals of 45 microns. During LEO growth, GaN grows perpendicular to the mask opening and spreads horizontally across the mask. The height and spread length of the grown GaN are both about 8 microns. A pn junction diode is formed on the LEO gallium nitride region. This pn junction diode includes an n-type GaN film with a thickness of 1 micron, and a Mg-doped p-type GaN film with a thickness of 0.5 micron grown on top of the n-type GaN film. The size of this pn junction diode is 2 microns x 20 microns.

In the GaN pn junction diode described in Kozodoy's paper, the reverse leakage current is lower in the low dislocation region (less than 10 ⁶ cm ^-2 ) than in the high dislocation region (about 4×10 ⁸ cm ^-2 ), This indicates that reverse breakdown is improved. However, the structure of the device in that paper is complex, and the device cannot be practically produced on low-dislocation regions. In the above-mentioned paper by Irokawa, the thickness of the GaN epitaxial layer is 3 micrometers, which is not enough for the carrier concentration to be 5×10 ¹⁶ cm ^-3 . The reverse blocking voltage of the PIN diode in Irokawa's paper is not high enough either.

The breakdown mechanism of a nitride semiconductor such as a diode is as follows. When the field strength at the Schottky junction or PN junction, which is the maximum field strength in the reverse bias state, is greater than a critical value, the impact ionization will cause a sharp increase in the reverse leakage current. This is the well-known phenomenon of breakdown. The ideal time for breakdown to occur is when the depletion layer is extended and the epitaxial layer is thick enough and the depletion layer is in the epitaxial layer even if the field strength at the junction reaches a critical value. However, if the thickness of the epitaxial layer is not sufficient to provide the carrier concentration, the entire thickness of the epitaxial layer will be exhausted before the field strength at the junction reaches the critical value (punchthrough), and the field strength at the junction will reach the critical value earlier, compared with Breakdown occurs at lower voltages than in the ideal case described above. In addition, since the depletion layer extends to the boundary surface between the epitaxial layer and the substrate, leakage current caused by defects in the boundary surface can reduce the reverse performance of the leakage current, possibly reducing the breakdown voltage. If punch through occurs due to these factors, the breakdown voltage will drop.

It is an object of the present invention to overcome these problems and provide a semiconductor element including a group III compound semiconductor layer including a structure for improving breakdown. Another object of the present invention is to provide an epitaxial substrate for the semiconductor element.

Contents of the invention

In one aspect, the present invention provides a semiconductor device including a Group III nitride semiconductor layer. The semiconductor element includes: (a) a free-standing gallium nitride substrate, the substrate includes a first surface and a second surface opposite to the first surface, the carrier concentration of which is greater than 1×10 ¹⁸ cm ^-3 ; (b) placing A first GaN epitaxial film on the first surface; (c) an ohmic electrode disposed on the second surface; and (d) a Schottky electrode disposed on the first GaN epitaxial layer. The thickness of the first GaN epitaxial film is at least 5 microns but not greater than 1000 microns. The carrier concentration of the first gallium nitride epitaxial film is at least 1×10 ¹⁴ cm ^-3 but not more than 1×10 ¹⁷ cm ^-3 . The semiconductor element is a Schottky diode.

In this Schottky diode, because the thickness of the first GaN epitaxial layer is at least 5 micrometers, but not more than 1000 micrometers, and because the carrier concentration of the first GaN epitaxial layer is at least 1 ×10 ¹⁴ cm ^-3 , but not greater than 1×10 ¹⁷ cm ^-3 , so the thickness and carrier concentration of the epitaxial layer can be designed to achieve an ideal breakdown effect without punch-through.

In another aspect, the present invention provides a semiconductor element including a Group III nitride semiconductor layer. The semiconductor element includes: (a) a gallium nitride support base plate, the base plate includes a first surface and a second surface opposite to the first surface, the carrier concentration of which is greater than 1×10 ¹⁸ cm ^-3 ; (b) placed on A first gallium nitride epitaxial layer on the first surface; (c) an ohmic electrode placed on the second surface; (d) a second nitrogen nitrogen layer placed on the first gallium nitride epitaxial layer and containing a p-type dopant a gallium nitride epitaxial layer; and (e) an ohmic electrode disposed on the second gallium nitride epitaxial layer. The gallium nitride support base has n-type conductivity. The thickness of the first GaN epitaxial layer is at least 5 microns, but not greater than 1000 microns. The carrier concentration of the first gallium nitride epitaxial layer is at least 1×10 ¹⁴ cm ^-3 but not more than 1×10 ¹⁷ cm ^-3 . The semiconductor element is a pn junction diode.

In this pn junction diode, because the thickness of the first GaN epitaxial layer is at least 5 μm but not more than 1000 μm, and because the carrier concentration of the first GaN epitaxial layer is at least 1×10 ¹⁴ cm ^-3 , but not greater than 1×10 ¹⁷ cm ^-3 , so the thickness and carrier concentration of the epitaxial layer can be designed to achieve an ideal breakdown effect without punchthrough.

In another aspect, the present invention provides a semiconductor element including a Group III nitride semiconductor layer. The semiconductor element includes: (a) a gallium nitride support base plate, the base plate includes a first surface and a second surface opposite to the first surface, the carrier concentration of which is greater than 1×10 ¹⁸ cm ^-3 ; (b) placed on A first gallium nitride epitaxial layer on the first surface; (c) a p-type semiconductor region placed in the first gallium nitride epitaxial layer; (d) an n-type semiconductor region placed in the p-type semiconductor region; (e ) a source electrode placed on the n-type semiconductor region; (f) a drain electrode placed on the second surface; (g) an insulating layer placed on the first gallium nitride epitaxial film; and (f) an insulating layer placed on the on the gate electrode. The thickness of the first GaN epitaxial layer is at least 5 microns, but not greater than 1000 microns. The carrier concentration of the first gallium nitride epitaxial layer is at least 1×10 ¹⁴ cm ^-3 but not more than 1×10 ¹⁷ cm ^-3 . The semiconductor elements are MIS transistors.

The MIS transistor has a structure including a source electrode placed on an n-type semiconductor region and a drain electrode placed on a second surface of a substrate, wherein current flows vertically from one electrode to the other. Because the thickness of the first GaN epitaxial layer is at least 5 μm but not more than 1000 μm, and because the carrier concentration of the first GaN epitaxial layer is at least 1×10 ¹⁴ cm ⁻³ but not more than 1× 10 ¹⁷ cm ^-3 , so the thickness and carrier concentration of the epitaxial layer can be designed to achieve the ideal breakdown effect without punch-through.

In the semiconductor device of the present invention, the p-type dopant in the p-type semiconductor region is preferably introduced by ion implantation. In addition, in the semiconductor element of the present invention, the n-type dopant in the n-type semiconductor region is preferably introduced by ion implantation.

In the semiconductor element of the present invention, the surface orientation of the first surface of the gallium nitride supporting substrate is preferably within a range of not more than +5 degrees of (0001) but at least -5 degrees of (0001). This provides a low dislocation GaN substrate.

In the semiconductor element of the present invention, the surface orientation of the first surface of the gallium nitride free-standing substrate is preferably +5 degrees not greater than (1-100) or (11-20) but at least (1-100) or (11 -20) within -5 degrees.

When this semiconductor element is used, the dislocations in the epitaxial layer are reduced, the reverse leakage current is reduced, and the reverse breakdown is improved.

In the semiconductor device of the present invention, the dislocation density of the first surface of the gallium nitride supporting base plate is preferably not greater than 1×10 ⁸ cm ^-2 .

When this semiconductor element is used, the dislocation density is low, so the dislocations in the epitaxial layer are reduced, and therefore, the reverse leakage current is reduced, and the reverse breakdown is improved.

In the semiconductor element of the present invention, the first surface of the gallium nitride supporting base plate preferably includes a first region having a dislocation density not greater than 1×10 ⁸ cm ⁻² and a second region having a dislocation density greater than that of the first region. area.

When this semiconductor element is used, the dislocations in the epitaxial layer formed on the region where the dislocation density is low are low. Therefore, the reverse leakage current of the semiconductor element is further reduced, and the reverse breakdown is improved.

In another aspect, the present invention provides an epitaxial substrate, which includes: (a) a gallium nitride free-standing substrate, the substrate includes a first surface and a second surface opposite to the first surface, the carrier concentration of which is greater than 1× 10 ¹⁸ cm ⁻³ ; and (b) a first gallium nitride epitaxial film disposed on the first surface. The thickness of the first GaN epitaxial film is at least 5 microns but not greater than 1000 microns. The carrier concentration of the first gallium nitride epitaxial film is at least 1×10 ¹⁴ cm ^-3 but not more than 1×10 ¹⁷ cm ^-3 .

When this epitaxial substrate is used, because the thickness of the first GaN epitaxial film is at least 5 microns but not more than 1000 µm, and because the carrier concentration of the first GaN epitaxial film is at least 1×10 ¹⁴ cm ^-3 , but not greater than 1×10 ¹⁷ cm ^-3 , so the thickness and carrier concentration of the epitaxial layer can be designed to achieve an ideal breakdown effect without punch-through. The epitaxial substrate thus provided can be used for a semiconductor element with improved reverse breakdown performance.

The epitaxial substrate of the present invention may also include a second gallium nitride epitaxial film disposed on the first gallium nitride epitaxial film and including a p-type dopant. When using the epitaxial substrate, the provided epitaxial substrate can be used for a pn junction diode with improved reverse breakdown performance. In addition, in the epitaxial substrate of the present invention, the p-type dopant is preferably introduced by ion implantation, or the p-type epitaxial layer is preferably formed by metal-organic vapor phase epitaxy.

The epitaxial substrate of the present invention may include: (c) a p-type semiconductor region disposed in the first gallium nitride epitaxial layer; and (d) an n-type semiconductor region disposed in the p-type semiconductor region. The first GaN epitaxial film and the GaN free-standing substrate preferably have n-type conductivity.

When the epitaxial substrate is used, the provided epitaxial substrate can be used for a transistor with improved reverse breakdown performance.

In the epitaxial substrate of the present invention, HVPE is preferably used to grow the first gallium nitride epitaxial film. Because the growth rate is fast, thick epitaxial films can be provided within a practical time. In the epitaxial substrate of the present invention, the second gallium nitride epitaxial film is preferably formed by metal organic vapor phase epitaxy. Using such an epitaxial substrate can provide a high-quality epitaxial film.

In the epitaxial substrate of the present invention, the surface orientation of the first surface of the gallium nitride free-standing substrate is preferably within a range of not more than +5 degrees of (0001) but at least -5 degrees of (0001).

Using such an epitaxial substrate can provide a low dislocation GaN substrate.

In the epitaxial substrate of the present invention, the surface orientation of the first surface of the gallium nitride free-standing substrate is preferably +5 degrees not greater than (1-100) or (11-20) but at least (1-100) or (11 -20) within -5 degrees.

Using such an epitaxial substrate can provide an epitaxial substrate for semiconductor elements with reduced dislocations in the epitaxial layer, reduced reverse leakage current, and improved reverse breakdown.

In the epitaxial substrate of the present invention, the dislocation density in the epitaxial layer is preferably not more than 1×10 ⁸ cm ^-2 .

When this epitaxial substrate is used, the dislocation density is low, so the dislocations in the epitaxial layer are reduced. Accordingly, it is possible to provide an epitaxial substrate for a semiconductor element with reduced reverse leakage current and improved reverse breakdown.

In the epitaxial substrate of the present invention, the first surface of the gallium nitride supporting base plate preferably includes a first region with a dislocation density not greater than 1×10 ⁸ cm ^-2 and a second region with a dislocation density greater than that of the first region. area.

When this epitaxial substrate is used, a semiconductor element can be formed on a region where the dislocation density is low, whereby dislocations in the epitaxial layer are further reduced. Accordingly, it is possible to provide an epitaxial substrate for a semiconductor element with reduced reverse leakage current and improved reverse breakdown.

The above and other objects, features and advantages of the present invention can be more clearly understood by reading the following description in conjunction with the accompanying drawings, in which similar symbols indicate the same components.

As described above, the present invention can provide a group III nitride device having a structure capable of improving reverse breakdown performance. An epitaxial substrate for such a semiconductor element can also be provided.

Description of drawings

FIG. 1 is a diagram showing a semiconductor element including a Group III nitride semiconductor layer of a first embodiment.

FIG. 2 is a graph showing the I-V characteristics of Sample A and Sample B. FIG.

FIG. 3A is a diagram for describing a breakdown phenomenon in a Schottky diode having a thick epitaxial film. FIG. 3B is a diagram for describing a breakdown phenomenon in a Schottky diode having a thin epitaxial film.

FIG. 4 is a graph showing the I-V characteristics of Sample A and Sample C. FIG.

FIG. 5 is a graph showing the I-V characteristics of Sample A and Sample D. FIG.

FIG. 6 is a graph showing I-V characteristics of Sample E, Sample F, and Sample G. FIG.

FIG. 7 is a diagram showing a semiconductor element including a Group III nitride semiconductor layer of a second embodiment.

FIG. 8 is a graph showing the I-V characteristics of Sample H. FIG.

FIG. 9A is a diagram showing a transistor of a fourth embodiment. FIG. 9B is a sectional view showing a section taken along line II-II in FIG. 9A .

10A-10C are diagrams showing production of an epitaxial substrate according to a fifth embodiment. 10D-10G are diagrams illustrating production of an epitaxial substrate.

FIG. 11A is a diagram showing an arrangement of high displacement regions and low displacement regions in a freestanding substrate. 11B is a diagram showing another arrangement of high displacement regions and low displacement regions in a freestanding substrate.

Detailed ways

The scope of the present invention is readily understood from the following description and accompanying drawings. A semiconductor element and an epitaxial substrate according to an embodiment of the present invention will be described below. Where possible, label identical parts with similar symbols.

(first implementation)

Fig. 1 shows a Group III nitride semiconductor element of the first embodiment of the present invention. The semiconductor element is a Schottky diode 11 . The Schottky diode 11 is equipped with a gallium nitride support substrate 13 , a gallium nitride epitaxial layer 15 , an ohmic electrode 17 and a Schottky electrode 19 . The gallium nitride supporting base 13 includes a first surface 13a and a second surface 13b opposite to the first surface, and its carrier concentration is greater than 1×10 ¹⁸ cm ⁻³ . A gallium nitride epitaxial layer 15 is placed on the first surface 13a. Ohmic electrodes 17 are placed on the second surface 13b. A Schottky electrode 19 is placed on the GaN epitaxial layer 15 . Thickness D1 of gallium nitride epitaxial layer 15 is 5 micrometers or more and 1000 micrometers or less. In addition, the carrier concentration of gallium nitride epitaxial layer 15 is 1×10 ¹⁴ cm ⁻³ or more and 1×10 ¹⁷ cm ⁻³ or less. When the carrier concentration is 1×10 ¹⁴ cm ^-3 or more, the "on" resistance can be kept low. When the carrier concentration is 1×10 ¹⁷ cm ⁻³ or less, the breakdown voltage can be increased.

In this Schottky diode 11, since the thickness of gallium nitride epitaxial layer 15 is 5 micrometers or more and 1000 micrometers or less, and because the carrier concentration is 1×10 ¹⁴ cm ⁻³ or larger and 1×10 ¹⁷ cm ⁻³ or smaller, so the epitaxial layer thickness and carrier concentration can be designed in a suitable manner to provide the desired breakdown effect without causing punch-through. Thereby, the breakdown voltage of the Schottky diode 11 can be increased.

The carrier concentration of the GaN substrate is higher than that of the epitaxial layer. As shown in FIG. 1, in a Schottky diode 11, an ohmic electrode 17 is placed on the entire second surface 13b. A Schottky electrode 19 is formed on a part of the surface of the epitaxial layer, for example, on a circle approximately in the center of the element. For the Schottky electrodes 19, materials such as nickel gold (Ni/Au) can be used, but Pt/Au or Au can also be used. The gallium nitride supporting substrate 13 and the gallium nitride epitaxial layer 15 serve as n-type conductors. The gallium nitride epitaxial layer 15 is grown homogeneously and epitaxially directly on the gallium nitride supporting substrate 13 . The thickness D2 of the gallium nitride supporting base 13 is preferably, for example, at least 100 micrometers, but not greater than 700 micrometers.

(first working example)

The (0001) plane GaN free-standing substrate produced by HPVE was prepared. Schottky diodes were produced using the following procedure. The carrier concentration of the n-type conductor GaN free-standing substrate is 3×10 ¹⁸ cm ^-3 , and the thickness is 400 microns. In this substrate, the average dislocation density was 5×10 ⁶ cm ^-2 . An n-type conductor epitaxial film with a carrier concentration of 5×10 ¹⁵ cm ⁻³ and a thickness of 20 μm was grown on a GaN freestanding substrate by HVPE to form an epitaxial substrate (hereinafter referred to as sample A). Ohmic electrodes are formed on the back surface of the substrate, and Schottky electrodes are formed on the epitaxial film. Ohmic electrodes are formed on the entire rear surface of the substrate after organic cleaning. To form an ohmic electrode, Ti/Al/Ti/Au (20nm/100nm/20nm/300nm) was formed by EB vapor deposition. After forming the ohmic electrode film, alloying was carried out at 600° C. for about 1 minute. A Schottky electrode was formed as a 500 nm gold film by resistance heating deposition. The shape of the Schottky electrode is a circle with a diameter of 200 microns. Before the formation of ohmic electrodes and Schottky electrodes, and before deposition, the surface of the epitaxial film was treated with HCl solution (hydrochloric acid 1:purified water 1) at room temperature for 1 minute.

An epitaxial film with a carrier concentration of 5×10 ¹⁵ cm ⁻³ and a thickness of 3 μm was grown on another GaN freestanding substrate by HVPE to form an epitaxial substrate (hereinafter referred to as sample B). Ohmic electrodes and Schottky electrodes were formed in the same manner as above.

FIG. 2 is a graph showing IV characteristics of Sample A and Sample B. FIG. In FIG. 2 , the characteristic curve C _A represents the characteristics of sample A, and the characteristic curve C _B represents the characteristics of sample B. FIG. 3A is a diagram for describing a breakdown phenomenon in a Schottky diode having a thick epitaxial film. FIG. 3B is a diagram for describing a breakdown phenomenon in a Schottky diode having a thin epitaxial film. The reverse breakdown voltage of sample B is less than that of sample A. The reason for this is as follows. As shown in Figure 3A, in Sample A, the epitaxial layer is thick enough that when the applied voltage increases, the depletion layer DepA reaches the boundary surface between the substrate and the epitaxial film before the Schottky electrode and the epitaxial film Impact ionization occurs near the boundary surface between them, resulting in the flow of reverse leakage current. This impact ionization determines the reverse breakdown voltage. As shown in Fig. 3B, in Sample B, the epitaxial film does not have sufficient thickness, and when the applied voltage is increased, a punch-through phenomenon occurs, in which the epitaxial surface is depleted before impact ionization occurs at the epitaxial surface below the Schottky electrode. Layer DepB reaches the interface between the substrate and the epitaxial film. This reduces the reverse breakdown voltage.

(second working example)

The (0001) plane GaN free-standing substrate produced by HPVE was prepared. The carrier concentration of the n-type conductor GaN free-standing substrate is 3×10 ¹⁸ cm ^-3 , and the thickness is 400 microns. In this substrate, the average dislocation density was 5×10 ⁵ cm ^-2 . An n-type conductor epitaxial film with a carrier concentration of 5×10 ¹⁵ cm ⁻³ and a thickness of 20 μm was grown on a GaN freestanding substrate by HVPE to form an epitaxial substrate (hereinafter referred to as sample C). A Schottky diode was formed on the epitaxial substrate by the method used in the first working example.

FIG. 4 is a graph showing the IV characteristics of Sample A and Sample C. FIG. In FIG. 4 , the characteristic curve C _A represents the characteristics of sample A, and the characteristic curve C _C represents the characteristics of sample C. In the GaN free-standing substrate of Sample A, the average dislocation density was 5×10 ⁶ cm ⁻² , whereas in the GaN free-standing substrate of Sample C, the average dislocation density was 5×10 ⁵ cm ⁻² . Compared with the reverse breakdown voltage of sample A, the reverse breakdown voltage of sample C is high. In other words, it can be considered that dislocations in the supporting base plate increase the reverse leakage current.

(third working example)

Preparation of (1-100) plane GaN free-standing substrate produced by HPVE. The carrier concentration of the n-type conductor GaN free-standing substrate is 3×10 ¹⁸ cm ^-3 , and the thickness is 400 microns. In this substrate, the average dislocation density was 5×10 ⁵ cm ^-2 . An n-type conductor epitaxial film with a carrier concentration of 5×10 ¹⁵ cm ^-3 and a thickness of 20 μm was grown on a GaN freestanding substrate by HVPE to form an epitaxial substrate (sample D). A Schottky diode was formed on the epitaxial substrate by the method used in the first working example.

FIG. 5 is a graph showing the IV characteristics of Sample A and Sample D. FIG. In FIG. 5, the characteristic curve _CA represents the characteristics of sample A, and the characteristic curve _CD represents the characteristics of sample D. Since the GaN free-standing substrate in sample A has a (0001) plane and the GaN free-standing substrate in sample D has a (1-100) plane, the reverse breakdown of sample C Wearability is improved. More specifically, when a gallium nitride film is epitaxially grown on a (1-100) plane, threading dislocations do not occur in the [0001] direction. So the leakage current in the Schottky diode is very small.

(fourth working example)

The (0001) plane GaN free-standing substrate produced by HPVE was prepared. The carrier concentration of the n-type conductor GaN free-standing substrate is 3×10 ¹⁸ cm ^-3 , and the thickness is 400 microns. HVPE was used to grow n-type conductor epitaxial films with a carrier concentration of 1×10 ¹⁷ cm ^-3 and a thickness of 10, 5 and 3 microns on GaN free-standing substrates to form epitaxial substrates (samples E, F, and G). Schottky diodes were formed on these epitaxial substrates by the method used in the first working example.

Figure 6 shows the IV characteristics of samples E, F and G described above. In FIG. 6, characteristic curves _CE , _CF , and _CG represent characteristics of samples E, F, and G, respectively. Sample E and Sample F exhibit approximately the same reverse breakdown voltage, but the reverse breakdown voltage of Sample G is smaller than that of Sample E and Sample F. In sample G, it is considered that a punch-through phenomenon occurs when the depletion layer in the epitaxial film reaches the boundary surface between the substrate and the epitaxial film when the applied voltage is increased. Thereby reducing the reverse breakdown voltage. Therefore, the thickness of the epitaxial film should be at least 5 micrometers.

In a drift layer (n layer) of a power conversion device such as a (Schottky) diode, the carrier concentration is preferably at least 1×10 ¹⁷ cm ⁻³ in order to improve breakdown performance. To prevent the punch-through phenomenon, it is important to adapt the epitaxial thickness to the carrier concentration. When the carrier concentration is 1×10 ¹⁷ cm ⁻³ , an epitaxial film having a thickness of 5 μm or more can provide an epitaxial film of sufficient thickness for high breakdown performance.

(second implementation)

FIG. 7 shows a semiconductor element including a Group III semiconductor layer of a second embodiment. This semiconductor element is a pn junction diode 31 . The pn junction diode 31 includes: a gallium nitride support base plate 33 ; a first gallium nitride epitaxial layer 35 ; a first ohmic electrode 37 ; a second gallium nitride epitaxial film 39 ; and a second ohmic electrode 41 . The gallium nitride supporting base 33 includes a first surface 33a and a second surface 33b opposite to the first surface 33a. Its carrier concentration is greater than 1×10 ¹⁸ cm ^-3 . The gallium nitride supporting base 33 has n-type conductivity. The thickness of the first GaN epitaxial layer 35 is at least 5 microns and not greater than 1000 microns. The carrier concentration of the first gallium nitride epitaxial layer 35 is at least 1×10 ¹⁴ cm ⁻³ and not more than 1×10 ¹⁷ cm ⁻³ . A first gallium nitride epitaxial layer 35 is placed on the first surface 33a. A first ohmic electrode (such as a cathode) 37 is placed on the second surface 33b. The second gallium nitride epitaxial film 39 is placed on the first gallium nitride epitaxial layer 35 and includes a p-type dopant. A second ohmic electrode (such as an anode) 41 is placed on the second GaN epitaxial film 39 .

When using this pn junction diode 31, because the thickness of the first gallium nitride epitaxial layer 35 is at least 5 microns and not more than 1000 microns, and because the carrier concentration of the first gallium nitride epitaxial layer 35 is at least 1×10 ¹⁴ cm ^-3 and not greater than 1×10 ¹⁷ cm ^-3 , so the ideal breakdown effect without punch-through can be achieved by properly designing the epitaxial layer thickness and carrier concentration.

The GaN supporting substrate 33 and the first GaN epitaxial layer 35 have n-type conductivity, and the second GaN epitaxial layer 39 has p-type conductivity. The carrier concentration of GaN free-standing substrate 33 is higher than that of epitaxial layer 35 . The carrier concentration of the first GaN epitaxial layer 35 is lower than the carrier concentration of the second GaN epitaxial layer 39 . Therefore, the depletion layer mainly extends toward the first GaN epitaxial layer 35 . The thickness and carrier concentration of the epitaxial layer 35 can be designed with the thickness and carrier concentration of the Schottky diode 11 of the first embodiment. The gallium nitride epitaxial layer 39 preferably has a carrier concentration of at least 1×10 ¹⁷ cm ⁻³ .

In the pn junction diode 31 , an ohmic electrode (cathode) 37 is placed on the entire second surface 33 b of the substrate 33 . The material used for the cathode can be, for example, Ti/Al/Ti/Au (20nm/100nm/20nm/300nm). The material used for the anode can be, for example, Ni/Au (50nm/100nm). The first GaN epitaxial layer 35 is grown homogeneously and epitaxially directly on the GaN supporting substrate 33 . The second GaN epitaxial layer 39 is homogeneously epitaxially grown directly on the first GaN epitaxial layer 35 . The thickness of the first GaN epitaxial layer 35 is preferably greater than the thickness of the second GaN epitaxial layer 39 . The thickness D3 of the second gallium nitride epitaxial layer is preferably, for example, at least 0.1 microns, but not greater than 10 microns.

(fifth working example)

The (0001) plane GaN free-standing substrate produced by HPVE was prepared. The carrier concentration of the n-type conductor GaN free-standing substrate is 3×10 ¹⁸ cm ^-3 , and the thickness is 400 microns. The dislocation density of this substrate was 5×10 ⁵ cm ^-3 . An n-type conductor epitaxial film with a carrier concentration of 5×10 ¹⁵ cm ^-3 and a thickness of 20 microns is grown on a GaN free-standing substrate by HVPE, thereby forming an epitaxial substrate. Then a p-type conductor GaN layer is formed by a metal-organic vapor phase epitaxial growth method, thereby forming an epitaxial substrate containing a pn junction. Doping was performed with 5×10 ¹⁹ cm ⁻³ Mg as a dopant, and its thickness was 1 μm. The carrier concentration was 1×10 ¹⁸ cm ^-3 . The p-type ohmic electrode was formed by the following method: first, the surface p-type layer was dry-etched into a mesa shape with a depth of about 2 micrometers using Cl ₂ -based RIE. Ni/Au resistance heating vacuum vapor deposition was carried out on the mesa, followed by heat treatment in nitrogen at 700 °C. The shape of the P-type electrode can be, for example, a circle with a diameter of 200 microns. The n-type ohmic electrode was formed by the following method: EB vacuum vapor deposition of Ti/Al/Ti/Au (20nm/100nm/20nm/300nm) on the entire back surface of the substrate, followed by heat treatment in nitrogen at 600°C for 1 minute ( Sample H). Figure 8 shows the IV characteristics of sample H. The reverse breakdown voltage shown in the figure is similar to that of sample C, which is a Schottky diode with the same structure.

(third implementation)

Fig. 9A is a diagram showing a transistor of the third embodiment. Fig. 9B is a cross-sectional view taken along line II-II in Fig. 9A. Group III nitride semiconductor MIS field effect transistor 71 includes: gallium nitride support substrate 53 ; gallium nitride epitaxial layer 55 ; p-type semiconductor region 57 ; n-type semiconductor region 59 ; source electrode 61 ; drain electrode 63 ; gate electrode 75 . The gallium nitride supporting base 53 includes: a first surface 53a and a second surface 53b opposite to the first surface 53a. The carrier concentration is greater than 1×10 ¹⁸ cm ^-3 . A gallium nitride epitaxial layer 55 is placed on the first surface 53a. A p-type semiconductor region 57 is placed on the gallium nitride epitaxial layer 55 . An n-type semiconductor region 59 is placed in the p-type semiconductor region 57 . The source electrode 61 is placed on the highly doped n-type semiconductor region 59 . The drain electrode 63 is placed on the second surface 53b. Gate electrode 75 is placed on insulating layer 77 formed on gallium nitride epitaxial layer 55 . The p-type semiconductor region 57 includes an extension portion 57b placed under the gate electrode 75 . The material used for the insulating layer may be a silicon oxide film, a silicon oxynitride film, a silicon nitride film, aluminum oxide, aluminum nitride, AlGaN, or the like. The GaN epitaxial layer 55 has a thickness of at least 5 microns but not greater than 1000 microns. The gallium nitride epitaxial layer 55 has a carrier concentration of at least 1×10 ¹⁴ cm ^-3 but not more than 1×10 ¹⁷ cm ^-3 .

The transistor 71 has a vertical structure, the source electrode 61 is placed on the n-type semiconductor region 59, the drain electrode 63 is placed on the second surface 53b of the substrate, and current flows from one electrode to the other. Because the thickness of the GaN epitaxial layer 55 is at least 5 microns but not more than 1000 microns, and because the carrier concentration of the GaN epitaxial layer 55 is at least 1×10 ¹⁴ cm ⁻³ but not more than 1×10 ¹⁷ cm ^-3 , so the ideal breakdown effect without punch-through can be achieved by properly designing the epitaxial layer thickness and carrier concentration.

By forming a p-type semiconductor region by ion implantation, it is possible to form a semiconductor element having a planar structure having a p-conductive semiconductor in a selected region. For the p-type dopant, magnesium or the like can be used. By forming an n-type semiconductor region by ion implantation, a semiconductor element having a planar structure having n-conductive semiconductor can be formed. For the n-type dopant, silicon or the like can be used. The p-type semiconductor region 57 electrically isolates the n-type semiconductor region 59 from the epitaxial layer 55 . The p-type semiconductor region 57 includes an extension portion 57b placed under the insulating film under the gate electrode. When a potential is applied to the gate electrode 75 , an n-type inversion layer is formed at the boundary surface between the insulating film and the p-type region 57 , and the potential reaches the epitaxial layer 55 from the n-type semiconductor region 59 through the carrier inversion layer. The depth of the p-type semiconductor region 57 is preferably at least 0.1 micrometers but not more than 3 micrometers. The carrier concentration of the surface portion of the p-type semiconductor region 57 is preferably at least 5×10 ¹⁷ cm ⁻³ . As shown in FIG. 9A , the branch 75 a of the gate electrode 75 is between the branches 61 a of the source electrode 61 . The corners of the electrodes 75, 61 are rounded to prevent breakdown.

In the semiconductor elements 11, 31, 71 of the first to third embodiments, the surface orientation of the first surface of the gallium nitride supporting substrate is preferably a (0001) plane (including crystallographically equivalent planes). As a result, a low-dislocation GaN substrate can be provided. In addition, in the semiconductor elements 11, 31, 71, the surface orientation of the first surface of the gallium nitride supporting substrate is preferably a (1-100) plane (including crystallographically equivalent planes) or a (11-20) plane (including crystallographically equivalent planes) or a (11-20) plane (including crystallographically equivalent planes). academic equivalent). These are preferably not more than +5 degrees from these crystal planes and not less than -5 degrees from these crystal planes in consideration of inconsistencies in surface orientation. When the semiconductor elements 11, 31, 71 are used, the dislocations in the epitaxial layer are reduced, the reverse leakage current is reduced, and the reverse breakdown is improved. In addition, when the semiconductor element 11, 31, 71 is used, the dislocation density on the first surface of the gallium nitride supporting base is preferably at least 1×10 ⁸ cm ⁻² . When the semiconductor element 11, 31, 71 is used, the low dislocation density reduces the dislocations in the epitaxial layer, therefore, the reverse leakage current is reduced and the reverse breakdown performance is improved. In addition, in the semiconductor element 11, 31, 71, the first surface of the gallium nitride supporting base plate preferably includes a first region with a dislocation density not greater than 1×10 ⁸ cm ⁻² and a dislocation density greater than the first region. The second area of density. When such semiconductor elements 11, 31, 71 are used, dislocations in the epitaxial layer can be further reduced if the semiconductor elements are formed in regions where the dislocation density is low. Therefore, the reverse leakage current is further reduced and the reverse breakdown performance is improved.

(fifth implementation)

10A-10C illustrate the production of an epitaxial substrate according to a fifth embodiment. As shown in FIG. 10A, a gallium nitride free-standing substrate 83 is prepared. The carrier concentration of the n-conductive GaN free-standing substrate 83 is greater than 1×10 ¹⁸ cm ⁻³ . As shown in FIG. 10B , an epitaxial film 85 is placed on the first surface 83 a of the GaN freestanding substrate 83 . GaN epitaxial film 85 has a thickness of at least 5 micrometers but not more than 1000 micrometers. Gallium nitride epitaxial film 85 may have n-type conductivity, for example, and its carrier concentration is at least 1×10 ¹⁴ cm ⁻³ but not more than 1×10 ¹⁶ cm ⁻³ . Thus, epitaxial substrate 81 is produced. The semiconductor elements of the first to third embodiments can be produced using such a substrate. The gallium nitride epitaxial film 85 is preferably grown by HVPE.

As shown in FIG. 10C , a Schottky electrode film 87 layer is placed on the epitaxial film 85 surface of the epitaxial substrate 81 , and an ohmic electrode film 89 layer is placed on the second surface 83 b of the substrate 83 . Since the thickness of GaN epitaxial film 85 is at least 5 µm but not more than 1000 µm, and because the carrier concentration is at least 1×10 ¹⁴ cm ^-3 but not more than 1×10 ¹⁷ cm ^-3 , it is possible to The thickness of the epitaxial layer and the carrier concentration are designed to achieve the ideal breakdown effect without punch-through when a potential is applied between the Schottky electrode film 87 and the ohmic electrode film 89 . This provides an epitaxial substrate for a semiconductor element with improved breakdown performance.

In this epitaxial substrate 81, a p-type semiconductor region may be formed on the gallium nitride epitaxial film 85, and an n-type semiconductor region may be formed in the p-type semiconductor region. This provides an epitaxial substrate for transistors with improved breakdown characteristics.

10D-10G illustrate the production of epitaxial substrates. Epitaxial substrate 81 is produced as shown in FIGS. 10D and 10E. As shown in FIG. 10F , a p-type GaN epitaxial film 93 is placed on the epitaxial substrate 81 to prepare the epitaxial substrate 91 . The gallium nitride epitaxial film 93 is preferably grown by metal organic vapor phase epitaxy. The carrier concentration of GaN epitaxial film 93 is higher than that of GaN epitaxial film 85 , so that the depletion layer is mainly formed on GaN epitaxial film 85 .

As shown in FIG. 10G, an ohmic electrode film 95 layer is placed on the epitaxial film 93 of the epitaxial substrate 91, and an ohmic electrode film 97 layer is placed on the second surface 83b. Because the thickness of the GaN epitaxial film 85 is at least 5 micrometers but not more than 1000 micrometers, and because the carrier concentration of the GaN epitaxial film 85 is at least 1×10 ¹⁴ cm ⁻³ but not more than 1×10 ¹⁷ cm ⁻³ , so the thickness of the epitaxial layer and the carrier concentration can be designed to achieve the ideal breakdown effect without punch-through when a potential is applied between the ohmic electrode film 95 and the ohmic electrode film 97 . This provides epitaxial substrate 91 for semiconductor elements with improved breakdown performance.

In the above-mentioned epitaxial substrates 81, 91, the epitaxial film 85 can be grown by HVPE, and the epitaxial film can be grown to a thickness of about 1000 micrometers at the maximum within a practicable time. A high-quality epitaxial film can be provided by metal organic vapor phase epitaxy using the epitaxial substrate 91 . In addition, when the epitaxial substrates 81 and 91 are used, the surface orientation of the first surface 83a of the gallium nitride free-standing substrate 83 is preferably the (0001) plane (including crystallographically equivalent planes). A low-dislocation GaN free-standing substrate can be provided with this epitaxial substrate. In addition, when epitaxial substrates 81 and 91 are used, the surface orientation of the first surface 83a of the gallium nitride free-standing substrate is preferably not greater than the (1-100) plane (including crystallographically equivalent planes) and the (11-20) plane ( +5 degrees including crystallographically equivalent planes) but at least -5 degrees from (1-100) planes (including crystallographically equivalent planes) and (11-20) planes (including crystallographically equivalent planes) within range. When the epitaxial substrates 81 and 91 are used, the dislocations in the epitaxial layer are reduced, the reverse leakage current is reduced, and the reverse breakdown is improved.

FIG. 11A is a diagram showing an arrangement of high and low dislocation regions on a GaN freestanding substrate. 11B is a diagram showing another arrangement of high and low dislocation regions on a GaN freestanding substrate. The first surface 82a of the gallium nitride freestanding substrate 82 used for the epitaxial substrates 81, 91 includes: a first region on which is shown a high dislocation region 82c having a larger thread dislocation density; a second region on which is shown There is a low dislocation region 82d with a smaller thread dislocation density. The high dislocation region 82c is surrounded by the low dislocation region 82d, and on the first surface 82a, the first region is randomly distributed in the second region. The overall thread dislocation density is, for example, not greater than 1×10 ⁸ cm ^-2 . When these epitaxial substrates 81, 91 are used, the low dislocation density reduces the dislocations in the epitaxial layer, therefore, the reverse leakage current is reduced and the reverse breakdown performance is improved.

In the gallium nitride freestanding substrate 84 shown in FIG. 11B , the first surface 84a includes: a first region showing a high dislocation region 84c having a larger thread dislocation density thereon; a second region showing thereon Low dislocation regions 84d with less threading dislocation density. The low dislocation region 82d extends along the high dislocation region 82c. As a result, on the first surface 84a, the first regions (stripe regions) and the second regions (stripe regions) are alternately arranged. Each low dislocation region 84d is separated from the other low dislocation region 82d by a high dislocation region 84c.

The thread dislocation density of the low dislocation region is at least 1×10 ⁸ cm ^-2 , and its thread potential density is higher than that of the first region, for example at least 1×10 ⁸ cm ^-2 . Dislocations in the epitaxial film can be further reduced by forming the semiconductor element on a region where the dislocation density is low. As a result, the reverse leakage current is further reduced and the reverse breakdown performance is improved.

Compared with semiconductor elements using silicon semiconductors, high reverse breakdown voltage semiconductor elements using gallium nitride semiconductors can provide higher reverse breakdown voltage and lower forward "on" resistance (forward "on" resistance).

The principles of the invention have been described in terms of preferred embodiments, but it will be understood by those of ordinary skill in the art that changes may be made in these arrangements and details without departing from the principles of the invention. The invention is not limited to the specific structures disclosed in these embodiments. For example, a normally-off transistor was described in the embodiment, but the present invention is not limited to this transistor. Therefore, the present invention includes the protection scope of the claims and corrections and changes made within the spirit of the claims.

Claims (19)

1. An epitaxial substrate comprising: a gallium nitride free-standing substrate comprising a first surface and a second surface opposite said first surface and having a carrier concentration greater than 1×10 ¹⁸ cm ⁻³ ; and a first gallium nitride epitaxial film disposed on the first surface; in: The thickness of the first gallium nitride epitaxial film is at least 5 microns but not greater than 1000 microns; and The carrier concentration of the first gallium nitride epitaxial film is at least 1×10 ¹⁴ cm ^-3 but not more than 1×10 ¹⁷ cm ^-3 . 2. The epitaxial substrate according to claim 1, comprising: a p-type semiconductor region disposed in said first gallium nitride epitaxial film; and an n-type semiconductor region disposed in said p-type semiconductor region; Wherein, the first GaN epitaxial film and the GaN free-standing substrate have n-type conductivity. 3. The epitaxial substrate of claim 1, further comprising a second gallium nitride epitaxial film disposed on the first gallium nitride epitaxial film and including a p-type dopant. 4. The epitaxial substrate according to claim 3, wherein the p-type dopant is introduced by ion implantation. 5. The epitaxial substrate according to claim 3, wherein the second gallium nitride epitaxial film is formed by a metal organic vapor phase epitaxial growth method. 6. The epitaxial substrate according to any one of claims 1-5, wherein the surface orientation of the first surface of the gallium nitride free-standing substrate is not more than +5 degrees of (0001) but at least (0001) within -5 degrees. 7. The epitaxial substrate according to any one of claims 1-5, wherein the surface orientation of the first surface of the gallium nitride free-standing substrate is not greater than (1-100) or (11-20 ) but at least within -5 degrees of (1-100) or (11-20). 8. The epitaxial substrate according to any one of claims 1-7, wherein the dislocation density of the first surface of the gallium nitride free-standing substrate is not greater than 1×10 ⁸ cm ^-2 . 9. The epitaxial substrate according to any one of claims 1-7, wherein the first surface of the gallium nitride free-standing substrate comprises a first surface with a dislocation density not greater than 1×10 ⁸ cm ^-2 . A region and a second region having a dislocation density greater than the dislocation density of the first region. 10. The epitaxial substrate according to any one of claims 1-9, wherein the first gallium nitride epitaxial film is grown by HVPE. 11. A semiconductor element comprising a Group III nitride semiconductor element, the semiconductor element comprising: a gallium nitride support base plate comprising a first surface and a second surface opposite said first surface and having a carrier concentration greater than 1×10 ¹⁸ cm ⁻³ ; a first gallium nitride epitaxial layer disposed on the first surface; an ohmic electrode placed on said second surface; and a Schottky electrode disposed on said first gallium nitride epitaxial layer; in: The thickness of the first gallium nitride epitaxial layer is at least 5 microns, but not greater than 1000 microns; The carrier concentration of the first gallium nitride epitaxial layer is at least 1×10 ¹⁴ cm ^-3 but not greater than 1×10 ¹⁷ cm ^-3 ; and The semiconductor element is a Schottky diode. 12. A semiconductor element comprising a Group III nitride semiconductor element, the semiconductor element comprising: a gallium nitride support base plate comprising a first surface and a second surface opposite said first surface and having a carrier concentration greater than 1×10 ¹⁸ cm ⁻³ ; a first gallium nitride epitaxial layer disposed on the first surface; an ohmic electrode placed on said second surface; a second epitaxial layer of gallium nitride disposed on the first epitaxial layer of gallium nitride and containing a p-type dopant; and an ohmic electrode disposed on the second gallium nitride epitaxial layer; in: The gallium nitride supporting base plate has n-type conductivity; The thickness of the first gallium nitride epitaxial layer is at least 5 microns, but not greater than 1000 microns; The first gallium nitride epitaxial film has a carrier concentration of at least 1×10 ¹⁴ cm ⁻³ but not more than 1×10 ¹⁷ cm ⁻³ ; and The semiconductor element is a pn junction diode. 13. A semiconductor element comprising a Group III nitride semiconductor element, the semiconductor element comprising: a gallium nitride support base plate comprising a first surface and a second surface opposite said first surface and having a carrier concentration greater than 1×10 ¹⁸ cm ⁻³ ; a first gallium nitride epitaxial layer disposed on the first surface; a p-type semiconductor region disposed in said first gallium nitride epitaxial layer; an n-type semiconductor region disposed in said p-type semiconductor region; a source electrode placed on said n-type semiconductor region; a drain electrode placed on said second surface; an insulating layer disposed on said first gallium nitride epitaxial film; and a gate electrode placed on said insulating layer; in: The thickness of the first gallium nitride epitaxial layer is at least 5 microns, but not greater than 1000 microns; The carrier concentration of the first gallium nitride epitaxial layer is at least 1×10 ¹⁴ cm ^-3 but not greater than 1×10 ¹⁷ cm ^-3 ; and The semiconductor element is an MIS transistor. 14. The semiconductor element according to claim 13, wherein the p-type dopant of the p-type semiconductor region is introduced by ion implantation. 15. The semiconductor element according to claim 13 or 14, wherein the n-type dopant in the n-type semiconductor region is introduced by ion implantation. 16. The semiconductor element according to any one of claims 11-15, wherein the surface orientation of the first surface of the gallium nitride supporting base plate is at +5 degrees not greater than (0001) but at least ( 0001) within -5 degrees. 17. The semiconductor device according to any one of claims 11-15, wherein the surface orientation of the first surface of the gallium nitride free-standing substrate is not greater than (1-100) or (11-20 ) but at least within -5 degrees of (1-100) or (11-20). 18. The semiconductor device according to any one of claims 11-17, wherein the dislocation density of the first surface of the gallium nitride supporting base plate is not greater than 1×10 ⁸ cm ^-2 . 19. The semiconductor device according to any one of claims 11-17, wherein the first surface of the gallium nitride supporting base comprises a first region with a dislocation density not greater than 1×10 ⁸ cm ^-2 and a second region having a dislocation density greater than said dislocation density of said first region.

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